Operating System Interview Questions: Top 75 Q&A with Examples

By / July 13, 2025

🌟 Introduction

💻 Operating Systems is one of the most crucial subjects for every computer science and IT student aiming to crack top job interviews. Whether you are preparing for campus placements, PSU exams, GATE, or technical coding interviews, mastering OS fundamentals is a must.

In this comprehensive guide, we bring you the Top 75 Operating System Interview Questions and Answers, each explained with simple and clear examples to strengthen your conceptual clarity. Inspired by the GeeksforGeeks (GFG) approach, this list covers all key topics including processes, threads, scheduling algorithms, memory management, and file systems.

🔖 Revise these important Q&A thoroughly to boost your confidence and crack your next OS round with ease.

1. What is an operating system?

Answer: Software that acts as an interface between user and hardware, managing resources and providing services.

Example: Windows, Linux, macOS.

2. Explain the functions of an operating system.

Answer:

  • Process management

     

  • Memory management

     

  • File system management

     

  • Device management

     

  • Security and access control

     

  • User interface

     

3. What is a process?

Answer: A process is an instance of a program running on a computer, performing some task, with its own allocated resources.

Explanation:

  • A program is a passive entity, such as a file containing a set of instructions (e.g. a C program stored on disk).

     

  • A process is an active entity, with:

     

    • Program code (text section)

       

    • Current activity (represented by the value of Program Counter and processor registers)

       

    • Process stack (contains temporary data like function parameters, return addresses, local variables)

       

    • Data section (contains global variables)

       

    • Heap (for dynamic memory allocation)

Example: Opening Chrome creates a process for it.

4. Define thread.

Answer: A thread is a unit of execution within a process that runs independently but shares the process’s resources.

Explanation:

  • It is sometimes called a lightweight process (LWP).

     

  • A thread shares the resources of its parent process, such as:

     

    • Code section

       

    • Data section

       

    • Open files

       

    • Heap memory

       

  • However, each thread has its own:

     

    • Program counter

       

    • Stack

       

    • Registers

Example:

In a web browser (process):

  • One thread handles rendering the page

     

  • Another thread handles user input

     

  • Another thread downloads data in the background

5. Difference between process and thread.

Feature

Process

Thread

Definition

An independent executing program with its own address space.

The smallest unit of execution within a process, also called a lightweight process.

Memory

Has separate memory space (code, data, heap, stack).

Share memory (code, data, heap) with other threads in the same process; only stack and registers are separate.

Overhead

High overhead for creation and context switching.

Low overhead for creation and context switching.

Communication

Processes use Inter Process Communication (IPC) like pipes, sockets, shared memory for communication.

Threads can communicate easily as they share the same address space.

Isolation

Processes are isolated from each other, providing better security and stability.

Threads are not isolated, as one thread can affect others by modifying shared data.

Crash Impact

If a process crashes, it does not affect other processes.

If a thread crashes, it can bring down the entire process.

Example

A browser and a text editor are two different processes.

In a browser process, one thread handles UI rendering, another handles network requests.

6. What is multitasking?

Answer: Multitasking allows you to run multiple programs at the same time on your computer, such as listening to music while typing notes and downloading files.

Explanation:

  • Task = Process or Program.

     

  • The CPU switches rapidly between tasks, giving the illusion that all are running simultaneously.

7. Explain multithreading.

Answer: Multithreading is like a chef preparing multiple dishes at the same time in one kitchen, where each dish is managed by a different thread working together efficiently.

Explanation:

  • Thread: Smallest unit of execution within a process.

     

  • In multithreading, a process is divided into multiple threads, each performing a specific task simultaneously.

8. What is multiprocessing?

Answer: Multiprocessing is like having multiple chefs in a kitchen, each cooking a different dish at the same time, thus finishing the entire meal preparation faster.

Explanation:

  • In multiprocessing systems, multiple CPUs work in parallel, processing different tasks at the same time.

     

  • Each CPU has its own registers, program counter, and execution unit, but they can share the system memory.

Example:

Modern laptops or servers with dual-core, quad-core, or octa-core processors use multiprocessing, where each core acts as a separate processor executing different processes concurrently.

9. What is a kernel?

Answer: The kernel is the brain of the operating system, controlling everything your computer does and making sure software and hardware work together smoothly.

Explanation:

  • It acts as a bridge between user applications and hardware.

     

  • Responsible for:

     

    • Process management (creating, scheduling, terminating processes)

       

    • Memory management (allocating and deallocating memory)

       

    • Device management (using device drivers to communicate with hardware)

       

    • File system management (reading/writing files)

       

    • System calls handling (interface between user applications and OS services)

       

10. Types of kernels.

Answer:

  • Monolithic kernel (Linux)

     

  • Microkernel (Minix)

     

  • Hybrid kernel (Windows)

     

11. What is context switching?

Answer: Context switching is like a teacher shifting attention from one student to another, saving the current student’s progress and resuming the other’s from where they left off.

Explanation:

  • When multitasking, the CPU needs to switch between processes or threads to give each a chance to execute.

     

  • Context switching involves:

     

    • Saving the current state (context) of the running process (e.g. program counter, registers) into its Process Control Block (PCB).

       

    • Loading the saved state of the next process to be executed from its PCB.

       

Example:

In multitasking OS:

  • Switching from a music player process to a word processor process involves a context switch.

12. Explain system call.

Answer: A system call is like asking the operating system for permission to do something that a normal program cannot do directly, such as accessing files or using network devices.

Explanation:

  • User programs run in user mode and do not have direct access to hardware or critical OS resources for security and stability.

     

  • When they need to perform privileged operations (e.g. reading a file, creating a process, communicating with hardware), they make a system call to the kernel, which executes it in kernel mode.

Example:

In C programming (Linux):

int fd = open(“data.txt”, O_RDONLY);

Here, open() is a system call to request the OS to open the file data.txt in read-only mode.

13. What is a deadlock?

Answer: Deadlock is when two or more processes wait forever for resources held by each other, causing them to be stuck indefinitely.

Explanation:

  • Processes are stuck forever, as none can continue execution.

     

  • It usually occurs in multi-processing or multi-threading systems when resources are shared.

     

Example:

Suppose Process A holds Resource 1 and waits for Resource 2, while Process B holds Resource 2 and waits for Resource 1.

Both are waiting for each other indefinitely, causing deadlock.

14. Conditions for deadlock.

Answer:

  • Mutual exclusion

     

  • Hold and wait

     

  • No preemption

     

  • Circular wait

     

15. Explain starvation.

Answer: Starvation is like being in a queue where new people keep coming in front of you, and you never get your turn.

Explanation:

  • Occurs when scheduling algorithms unfairly allocate CPU or resources to certain processes over others.

     

  • The starving process remains ready to execute but never gets CPU time or required resources due to continuous preemption by higher-priority processes.

Example:

In a priority-based CPU scheduling system, if new high-priority processes keep entering, a low-priority process may never get scheduled, resulting in starvation.

16. What is aging?

Answer: Aging is like rewarding someone waiting in a queue for a long time by moving them closer to the front, so they eventually get their turn.

Explanation:

  • In scheduling algorithms (especially priority scheduling), low-priority processes may starve if high-priority processes keep arriving.

     

  • Aging solves this problem by: 

Increasing the priority of waiting processes at regular intervals.

Eventually, the waiting process’s priority becomes high enough to be scheduled and executed.

Example:

Suppose a process has priority 10 (low). Every 5 minutes its priority increases by 1.

After sufficient time, its priority becomes high enough to compete with other processes, preventing starvation.

17. Define semaphore.

Answer: A semaphore is like a traffic signal allowing a certain number of cars (processes) to pass, preventing collisions (race conditions) at intersections (shared resources).

Explanation:

  • Used to solve critical section problems by preventing race conditions.

     

  • It is an integer variable that is accessed only through two atomic operations:

     

    1. wait (P or down)

       

    2. signal (V or up)

Example:

In the Producer-Consumer Problem:

  • Semaphores are used to keep track of empty slots and full slots in a buffer to synchronize producer and consumer operations.

18. Difference between mutex and semaphore.

Feature

Mutex

Semaphore

Definition

Mutual Exclusion lock, used to provide exclusive access to a shared resource.

A synchronization tool that controls access to resources by maintaining a count.

Value range

Can only be 0 or 1 (binary lock).

Can be any non-negative integer (counting semaphore) or binary (0 or 1).

Usage

Used only for mutual exclusion (locking mechanism).

Used for mutual exclusion and synchronization (e.g. controlling access to multiple resources).

Ownership

Has ownership – only the thread that locked (acquired) it can unlock (release) it.

No ownership – any process can signal (release) it, not necessarily the one that waited (acquired) it.

Example scenario

Allowing only one thread to write to a file at a time.

Limiting access to 3 printers among multiple processes (counting semaphore) or acting as a mutex (binary semaphore).

Deadlock risk

If not released by the owning thread, it can cause deadlock.

Improper usage can cause deadlock or starvation.

Implementation

Acts as a lock/unlock mechanism.

Acts as a counter with wait() and signal() operations.

Atomic operations

lock() and unlock().

wait() (P) and signal() (V).

19. What is a critical section?

Answer: A critical section is like a bathroom in a house – only one person (process) should use it at a time to avoid conflict.

Explanation:

  • When multiple processes or threads access shared data or resources concurrently, inconsistent results may occur if their execution is not controlled.

     

  • The critical section problem is ensuring that when one process is executing its critical section, no other process is allowed to execute theirs simultaneously.

Example:

Suppose two threads are updating the same bank account balance simultaneously. If both read and write without coordination, the final balance may be incorrect.

Updating the balance should happen in the critical section protected by locks or synchronization mechanisms.

20. Explain busy waiting.

Answer: Busy waiting is like repeatedly knocking on a closed door until it opens, instead of sitting and waiting for someone to call you in.

Explanation:

  • In busy waiting, the process stays in the “ready” state, running in a loop to check if it can enter its critical section or acquire a lock.

     

  • It does not get blocked or suspended; instead, it consumes CPU time while waiting.

Example:

while (lock == 1) {

  // busy wait – continuously checking until lock becomes 0

}

lock = 1; // acquire lock

Here, the process keeps checking the value of lock in a loop until it is free.

21. Define paging.

Answer: Paging is like dividing a book into equal-sized pages and placing them in available slots on a shelf. The pages do not need to be in order physically, as an index (page table) keeps track of where each page is.

Explanation:

  • When a process is executed:

     

    • Its pages are loaded into available frames in physical memory.

       

  • Page size = Frame size, so pages fit perfectly into frames.

     

  • Eliminates external fragmentation, but internal fragmentation may occur if the last page is not completely used.

     

Example:

If page size is 4KB, and process has 10KB:

  • The process will have 3 pages.

     

  • These 3 pages can be loaded into any 3 free frames in memory, not necessarily contiguous.

22. What is segmentation?

Answer: Segmentation is like dividing a book into chapters of different lengths based on content, instead of cutting it into equal-sized pages like paging.

Explanation:

  • Unlike paging (which divides memory into fixed-size pages), segmentation divides programs based on logical divisions.

     

  • Each segment has:

     

    • Segment number

       

    • Segment length

       

  • The segment table maintains the base address and limit (length) of each segment.

     

Example:

A program can be divided into:

  • Code segment

     

  • Data segment

     

  • Stack segment

     

  • Heap segment

     

Each of these segments is loaded into memory separately and can be placed at different locations.

23. Difference between paging and segmentation.

Paging

Segmentation

Fixed-size blocks (pages & frames)

Variable-size blocks (segments)

Divides physical and logical memory into equal-sized pages/frames

Divides logical memory into logical units (segments)

Causes internal fragmentation

Causes external fragmentation

No logical meaning to division

Divisions are meaningful (functions, arrays, etc.)

24. What is virtual memory?

Answer: Virtual memory is like having an extra storage room (disk) for things you don’t need immediately in your office (RAM), giving you the feeling of having a bigger workspace.

Explanation:

  • Combines physical memory (RAM) and secondary storage (disk) to create a larger logical memory space.

     

  • Uses paging or segmentation with demand paging to load only required parts of a program into RAM.

Example:

Suppose your system has 4GB RAM, but you open applications needing 8GB.

Virtual memory allows this by storing inactive pages on disk, swapping them in and out of RAM as needed.

25. Explain page replacement.

Answer: Page replacement is like removing an old book from a full shelf to place a new one, based on a strategy (e.g. remove least recently read book).

Why is it needed?

  • In virtual memory systems, RAM may not have enough space to hold all the pages of active processes.

     

  • When a page fault occurs and no free frame is available, page replacement is triggered.


🌟 Conclusion

✔️ Strong fundamentals in Operating Systems empower you to confidently tackle both theory-based questions and practical coding interview scenarios. This curated list of Top 75 OS Interview Questions and Answers, explained with detailed examples, ensures you cover every major concept – from process management to memory allocation, scheduling to file systems.

💡 Keep practicing, revise these questions regularly, and enhance your problem-solving approach. With consistent preparation, you will undoubtedly ace your upcoming tech interviews and stand out as a strong candidate in your dream companies.


👉 Explore related interview guides to level up your preparation:

 

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